In this tutorial we are going to learn how to set up an Email server using Postfix, Dovecot and Squirrelmail on CentOS 7.x. We will be using Postfix for SMTP (Simple Mail Transfer Protocol), Dovecot for POP/IMAP and Squirrelmail as webmail client to send or receive emails. We will also learn to setup MX records which is important to route the emails.

Requirements

The requirements for setting up email server will simply be a VPS or dedicated server with a fresh CentOS 7.x install and also a static IP address. In this tutorial we will be using a root account to execute the commands. If you are logged in as non-root user, use sudo command at the start of all the commands or you can execute su command to login as root user.

Setting up DNS

It is very important to setup DNS records, specifically MX records in your domain control panel. Login to your domain control panel and change your DNS settings to add these following MX records entries. A typical MX record will look like this.

    Type   Host    Destination             Priority   TTL
    MX     @       mail.yourdomain.com     10         3600
    A      mail    192.168.0.1                        3600

Where MX is the type of record, MX stands for Mail Exchangers. Next is the value for host, you can either enter your domain name or you can also use @ which represents the value of the zone name which is same as your domain name. Next you will have to choose the destination, you will need to enter the hostname or FQDN of your mail server.

The next value is the priority. The lowest number is priority. For example 0 will have the highest priority and 20 will have a lower priority. Priority is used because we can add multiple MX records for a single domain, mail is forwarded to the server having highest priority. If the server having highest priority is not available then mail will be forwarded to the server having second highest priority. Next is TTL or Time to Live, it should be set to 3600.

It is very important that you also setup an A record for your hostname of the mail server FQDN. Again select the type as A record, host should be the hostname you are using in your FQDN, for example in this case we have used the hostname as mail.yourdomain.com, hence our host will be mail. Next, at destination, enter the IP address of your server. A records does not have priority option hence, you will only need to provide TTL.

Once you have configured your DNS settings, you will need to wait some time so that DNS gets propagated. It usually takes around two hours these days. Once propagated, you can check your MX records here.

Until your DNS gets propagated, you can continue with the installation.

Installing Postfix

Login to your server and run the following command to update the repository and packages available in your system.

    yum -y update

Now update the hostname of your system to the FQDN you want to use with your mail server. Run the following command to change your hostname.

    hostname mail.yourdomain.com

You can replace the hostname according to your choice, but it should be same as the FQDN which we have used in our DNS settings.

Now add the hostname entry in the host files of your system. Edit /etc/hosts files using your favorite editor. For example if your are using nano then you will need to run the following command.

    nano /etc/hosts

You will see two lines of entries in there, append your server IP address followed by hostname at the end of the file. It should look like the one shown below.

    127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
    ::1       localhost localhost.localdomain localhost6 localhost6.localdomain6
    104.36.18.239 mail.yourdomain.com

Now we can install Postfix, enter the following command to do so.

    yum -y install postfix

Before configuring postfix we will need to configure SSL which will be used to encrypt and secure the emails.

    mkdir /etc/postfix/ssl
    cd /etc/postfix/ssl

Now we will have to create SSL certificates. If you do not have openssl installed you can install it using the following command.

    yum -y install openssl

Now run the following command to create certificate and key files.

    openssl req -x509 -nodes -newkey rsa:2048 -keyout server.key -out server.crt -nodes -days 365

Now you will be asked some information which is to be added into your CSR (Code Signing Request). You will be asked your country name in two letters, for example consider IN for India. Then you will be asked about the state or province. Then you will be asked about your city and organization. Finally, a common name of your server and your email address. If you want to leave some details blank use full stop or period ( . ) sign. You can also enter the default values just by pressing enter. Example output is given below.

    Generating a 2048 bit RSA private key
    ..........................+++
    ...........................+++
    writing new private key to 'server.key'
    -----
    You are about to be asked to enter information that will be incorporated
    into your certificate request.
    What you are about to enter is what is called a Distinguished Name or a DN.
    There are quite a few fields but you can leave some blank
    For some fields there will be a default value,
    If you enter '.', the field will be left blank.
    -----
    Country Name (2 letter code) [XX]:IN
    State or Province Name (full name) []:RAJ
    Locality Name (eg, city) [Default City]:
    Organization Name (eg, company) [Default Company Ltd]:
    Organizational Unit Name (eg, section) []:My Unit
    Common Name (eg, your name or your server's hostname) []:mail.rackvoucher.com
    Email Address []:me@liptanbiswas.com

This will generate the key file and certificates and will save then in /etc/postfix/ssl directory.

Now edit postfix configuration file which can be found at /etc/postfix/main.cf, with your favorite editor.

    nano /etc/postfix/main.cf

and append these lines at the end of the file.

    myhostname = mail.yourdomain.com
    mydomain = yourdomain.com
    myorigin = $mydomain
    home_mailbox = mail/
    mynetworks = 127.0.0.0/8
    inet_interfaces = all
    inet_protocols = all
    inet_interfaces = localhost
    mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
    smtpd_sasl_type = dovecot
    smtpd_sasl_path = private/auth
    smtpd_sasl_local_domain =
    smtpd_sasl_security_options = noanonymous
    broken_sasl_auth_clients = yes
    smtpd_sasl_auth_enable = yes
    smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination
    smtp_tls_security_level = may
    smtpd_tls_security_level = may
    smtp_tls_note_starttls_offer = yes
    smtpd_tls_loglevel = 1
    smtpd_tls_key_file = /etc/postfix/ssl/server.key
    smtpd_tls_cert_file = /etc/postfix/ssl/server.crt
    smtpd_tls_received_header = yes
    smtpd_tls_session_cache_timeout = 3600s
    tls_random_source = dev:/dev/urandom

Now open another configuration file /etc/postfix/master.cf using your favorite editor.

    nano  /etc/postfix/master.cf

and find the following lines in the configuration file.

    # service type  private unpriv  chroot  wakeup  maxproc command + args
    #               (yes)   (yes)   (yes)   (never) (100)
    # ==========================================================================
    smtp      inet  n       -       n       -       -       smtpd

Now add the following lines at just below these lines.

    submission     inet  n       -       n       -       -       smtpd
      -o syslog_name=postfix/submission
      -o smtpd_sasl_auth_enable=yes
      -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
      -o milter_macro_daemon_name=ORIGINATING
    smtps     inet  n       -       n       -       -       smtpd
      -o syslog_name=postfix/smtps
      -o smtpd_sasl_auth_enable=yes
      -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
      -o milter_macro_daemon_name=ORIGINATING

Installing Dovecot

Now install Dovecot using the following command:

    yum -y install dovecot

Once Dovecot is installed, edit the following file using your favorite editor.

    nano /etc/dovecot/conf.d/10-master.conf

and find the following lines:

    # Postfix smtp-auth

Now Append the following lines, just below these lines:

    # Postfix smtp-auth
    unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
    }

Now open another configuration using the following command.

    nano /etc/dovecot/conf.d/10-auth.conf

and find the following lines.

    # Space separated list of wanted authentication mechanisms:
    #   plain login digest-md5 cram-md5 ntlm rpa apop anonymous gssapi otp skey
    #   gss-spnego
    # NOTE: See also disable_plaintext_auth setting.
    auth_mechanisms = plain

Append login at the end of the line auth_mechanisms = plain to make it look like

    auth_mechanisms = plain login

Again edit /etc/dovecot/conf.d/10-mail.conf file using your favorite editor.

    nano /etc/dovecot/conf.d/10-mail.conf

Find the following lines

    # 
    #
    #mail_location =

Now add the following line just below these lines:

    mail_location = maildir:~/mail

Now edit /etc/dovecot/conf.d/20-pop3.conf using your favorite editor.

    nano /etc/dovecot/conf.d/20-pop3.conf

and find the following lines.

    #pop3_uidl_format = %08Xu%08Xv

Uncomment the above line to make it look like as shown below.

    # Note that Outlook 2003 seems to have problems with %v.%u format which was
    # Dovecot's default, so if you're building a new server it would be a good
    # idea to change this. %08Xu%08Xv should be pretty fail-safe.
    #
    pop3_uidl_format = %08Xu%08Xv 

Now restart postfix, and dovecot using the following command.

    systemctl restart postfix
    systemctl enable postfix
    systemctl restart dovecot
    systemctl enable dovecot

Now if you have a firewall running you will need to allow port number 25, 587, 465, 110, 143, 993, 995 and 80. All the ports except 80 are used to send and receive emails and port 80 is used to make HTTP connections. HTTP connections will be used to access Squirrelmail using web interface.

To unblock all these ports from firewall, run the following commands.

    firewall-cmd --permanent --add-service=smtp
    firewall-cmd --permanent --add-port=587/tcp
    firewall-cmd --permanent --add-port=465/tcp
    firewall-cmd --permanent --add-port=110/tcp
    firewall-cmd --permanent --add-service=pop3s
    firewall-cmd --permanent --add-port=143/tcp
    firewall-cmd --permanent --add-service=imaps
    firewall-cmd --permanent --add-service=http
    firewall-cmd --reload

Testing Postfix and Dovecot

You can check if Postfix is working by executing the following command in your terminal.

    telnet mail.yourdomain.com smtp

If you do not have telnet installed, then you can run the following command to install telnet.

    yum -y install telnet

Once you are connected using telnet you will see following output.

    Trying ::1...
    Connected to localhost.
    Escape character is '^]'.
    220 mail.rackvoucher.com ESMTP Postfix

Now you can also send email using telnet. Use the following command to enter the sender username.

    mail from:

To enter the email of recipient, you can use the use the following command.

    rcpt to:

To enter the body of email, enter the following command.

    data

Once you have entered your message, enter dot (.) to finish the message. Finally enter quit to exit telnet.

To test Dovecot, enter the following command.

    telnet mail.yourdomain.com pop3

You will see following output,

    Trying 104.36.18.239...
    Connected to mail.rackvoucher.com.
    Escape character is '^]'.
    +OK Dovecot ready.

It tells that Dovecot is working fine, you can login to your mail account by providing login command, then use pass command to enter your password. To view the mails in your account, use retrcommand.

    user mailuser
    +OK
    pass Password
    +OK Logged in.
    retr
    -ERR There's no message 1.
    quit
    +OK Logging out.
    Connection closed by foreign host.

Installing Squirrelmail

As we have both Postfix and Dovecot working, we can now install Squirrelmail to your server. Squirrelmail does not comes with the default CentOS repository, hence you will need to add EPEL repository into your system using the following command.

    yum -y install epel-release

Now you can install Squirrelmail using the following command.

    yum -y install squirrelmail

After installing Squirrelmail you can configure it by running the configuration script.

    cd /usr/share/squirrelmail/config/
    ./conf.pl

You will see following output.

    SquirrelMail Configuration : Read: config.php (1.4.0)
    ---------------------------------------------------------
    Main Menu --
    1.  Organization Preferences
    2.  Server Settings
    3.  Folder Defaults
    4.  General Options
    5.  Themes
    6.  Address Books
    7.  Message of the Day (MOTD)
    8.  Plugins
    9.  Database
    10. LanguagesD.  Set pre-defined settings for specific IMAP serversC   Turn color off
    S   Save data
    Q   Quit
Command >>

In Option 1 you can change your organisation preferences. It is recommended to change it according to your organisation in production environment. If you choose option 1, you will see following output.

    SquirrelMail Configuration : Read: config.php (1.4.0)
    ---------------------------------------------------------
    Organization Preferences
    1.  Organization Name      : SquirrelMail
    2.  Organization Logo      : ../images/sm_logo.png
    3.  Org. Logo Width/Height : (308/111)
    4.  Organization Title     : SquirrelMail $version
    5.  Signout Page           : 
    6.  Top Frame              : _top
    7.  Provider link          : http://squirrelmail.org/
    8.  Provider name          : SquirrelMailR   Return to Main Menu
    C   Turn color off
    S   Save data
    Q   Quit
Command >>

Change the organisation name, logo and title according to your need. Once done, return to main menu using R command. In main menu choose option 2 for Server settings.

    SquirrelMail Configuration : Read: config.php (1.4.0)
    ---------------------------------------------------------
    Server SettingsGeneral
    -------
    1.  Domain                 : localhost
    2.  Invert Time            : false
    3.  Sendmail or SMTP       : SendmailA.  Update IMAP Settings   : localhost:143 (uw)
    B.  Change Sendmail Config : /usr/sbin/sendmailR   Return to Main Menu
    C   Turn color off
    S   Save data
    Q   Quit
Command >> 1

Change your domain name by selecting option 1.

    The domain name is the suffix at the end of all email addresses.  If for example, your email address is jdoe@example.com, then your domain would be example.com.
[localhost]: yourdomain.com

Now change your MTA by selecting the 3rd option.

    SquirrelMail Configuration : Read: config.php (1.4.0)
    ---------------------------------------------------------
    Server SettingsGeneral
    -------
    1.  Domain                 : rackvoucher.com
    2.  Invert Time            : false
    3.  Sendmail or SMTP       : SendmailA.  Update IMAP Settings   : localhost:143 (uw)
    B.  Change Sendmail Config : /usr/sbin/sendmailR   Return to Main Menu
    C   Turn color off
    S   Save data
    Q   QuitCommand >> 3You now need to choose the method that you will use for sending
    messages in SquirrelMail.  You can either connect to an SMTP server
    or use sendmail directly.
1. Sendmail 2. SMTP Your choice [1/2] [1]: 2

Now save your setting by giving S command and finally quit using Q command.

Now you will need to install the Apache web server, so that we can access Squirrelmail using web interface. Run the following command to install Apache web server.

    yum -y install httpd

Once Apache is installed, edit the configuration file to add a new virtual host.

    nano /etc/httpd/conf/httpd.conf

Now add the following lines at the end of the file.

    Alias /webmail /usr/share/squirrelmailOptions Indexes FollowSymLinks
        RewriteEngine On
        AllowOverride All
        DirectoryIndex index.php
        Order allow,deny
        Allow from all

Save the file and start and enable Apache web server using the following commands.

    systemctl start httpd
    systemctl enable httpd

Now create email users, run the following command to add a user.

    useradd -m liptan -s /sbin/nologin
    passwd liptan

Theabove command will add a new user liptan and the attribute -s /sbin/nologin will deny login using SSH. Last command will create a password for the new user.

Now you can browse Squirrelmail by going to following link into the browser.

    http:///webmail

You will see following screen.

Once you login you will see the following webmail interface.

You can now read your emails and send emails through this interface.

Conclusion

In this tutorial we have installed an email server, using Postfix, Dovecot and the Squirrelmail webmail client. You can now successfully deploy the email server and start sending via the mail server.

[출처] https://hostpresto.com/community/tutorials/how-to-setup-an-email-server-on-centos7/